1. Field of the Invention
This invention relates to an electrophotographic photosensitive sensor and more particularly to a sensor made mainly of non-monocrystalline silicon and/or germanium.
2. Description of the Prior Art
As described in U.S. Pat. No. 4,265,991, an electrophotographic photosensitive sensor made mainly of amorphous silicon, has various advantages as compared to the one made mainly of selenium or cadmium sulfide. For example, it has a higher photoconductivity; it has a sperior heat resisting property and a sperior abrasion resisting property; and it is harmless to its environment. In case of an electrophotographic photosensitive sensor having a photoconductive layer made mainly of amorphous silicon, however, it is desired that the photoconductive layer is made to have a higher dark resistivity and be sufficiently thick for holding electrostatic charges long enough for a developing process.
Recently, much attention has been paid to amorphous or microcrystalline silicon in the technical field concerning a solar cell. Generally, it is expected that this material will improve the sensitivity of electrophotographic photosensitive sensors. Microcrystalline silicon has a small optical energy band gap and a high dark conductivity, e.g., 10.sup.-3 -10.sup.-4 .OMEGA..sup.-1 cm.sup.-1. Therefore, charge carriers generated by irradiation of such an electrophotographic photosensitive sensor having a photoconductive layer made mainly of microcrystalline silicon, can move readily in the photoconductive layer, but it becomes difficult to hold enough surface electrostatic charges.
Although electrophotographic photossensitive sensors are now used not only in an electrophotographic machine but also in a laser printer, an LED printer, an intelligent copier and the like, it is desirable that such a photosensitive device should have an ample sensitivity even with respect to light of a longer wavelength. For example, a laser printer utilizes a laser beam with a wavelength of 780nm emitted by a semiconductor laser of an InGaAsP system. However, although a photoconductive layer of the prior art shows a high sensitivity in the wavelength range of 400-700nm, the sensitivity drastically decreases in the range longer than 750nm. Therefore, if the wavelength of the laser becomes longer due to a temperature change, the sensitivity of the photoconductive layer will be influenced and the copied image will become indistinct or blurred. Generally, an electrophotographic photosensitive sensor is provided with a blocking layer between a conductive substrate and a photoconductive layer. The blocking layer prevents so-called minority carriers in the blocking layer from being injected into the photoconductive layer from the substrate. Such injection of minority carriers causes dark decay of electrostatic charges on the surface of the photoconductive layer. A conventional blocking layer made mainly of amorphous silicon containing hydrogen (a-Si:H) is doped with a dopant of a p-type or an n-type. The doped blocking layer prevents so-called minority carriers in the blocking layer from being injected into the photoconductive layer from the substrate.
When the optical energy band gap in a blocking layer of amorphous silicon is set much larger than 1.7eV, carriers tend to be trapped in localized state levels between the band gaps and thus the residual charge becomes rather large. Therefore, the band gap in a blocking layer is generally set about 1.7eV. However, since a blocking layer having a band gap of 1.7eV does not have enough dark resistivity, it can not contribute much to holding the surface electrostatic charges on the photoconductive layer. Further, adhension between a blocking layer of amorphous silicon and a substrae of an aluminum alloy is not so good. Thus, if the blocking layer is deposited thicker than 10 .mu.m, it will tend to easily peel off. Still further, when a blocking layer of amorphous silicon receives a laser beam of a longer wavelength, interference is caused between the incident beam and a beam reflected by the substrate, since the blocking layer with the relatively large band gap does not effectively absorb the laser beam.